The demand for high quality journal bearings with improved dimensional precision, such as needed for greater recording track densities of magnetic disks or hostile environments, requires that cylindricity of mating surfaces be held within narrower tolerances in the range of a few microinches. Gas bearings, particularly the self-acting type, are well-suited for such applications, but the dimensional and geometric precision of production components is difficult to achieve since the surfaces cannot be reliably characterized.
In order to improve the positional accuracy, enable starting and stopping without use of a jacking gas, and increase the rotational velocities, the mating bearing surfaces must be known to be formed with a high degree of precision, exhibiting little, if any, eccentricity, and the bearing elements must certainly be capable of inspection as to these required tolerances. Although the extreme accuracy has been necessary, there has not been available a reliable technique for precisely measuring variations from true cylindricity of either a rotating shaft or its bearing under dynamic conditions and throughout the entire mating surfaces. Finishes and run-out tolerances can easily be specified but there is needed a technique to assure the dimensional quality of journal elements and their associated bearing surfaces.
Various gaging arrangements for measuring bearing deviations from cylindricity have been proposed. One of the more reliable techniques has been to use non-contact capacitive gaging. Present day systems can measure in the microinch range and provide direct read-out of sensed distances. A prior system that has been used to measure ovality of a hollow cylinder, detects capacitance changes at diametrically opposite points as a probe is slowly moved internally along a pipe, as is shown in U.S. Pat. No. 3,867,691. A similar arrangement is shown in U.S. Pat. No. 4,295,092 where a probe detects changes in capacitance due to dimensional changes in a corroded pipe wall. Another method has been to measure internal dimensions of a mold by using a probe having multiple capacitors, as in U.S. Pat. No. 4,352,060. In this instance, the probe carrying the capacitors is moved along a guide rail and each capacitor senses the immediately adjacent wall distance as the probe progresses.
In the known art, there is no method disclosed for determining with precision the variation in distance between the surface of a rotating member relative to that of a stationary member along a plurality of planes and under dynamic conditions. The prior art further neglects establishment of a true functional center from which measurements can be referenced. Measurements of diameters or radii have been merely approximations, even with a highly sensitive capacitive instrument, because the probes are not steadily supported on, nor positioned about an accurate and reliable axis. Nor have the known techniques been able to provide high resolution mapping of the entire surface of interest in an efficient and inexpensive manner. The prior art is further devoid of teachings to determine to the same accuracy the squareness of end surfaces of an apparent right circular cylinder under dynamic thrust loading.